The present invention relates in general to display systems and to data processing systems which incorporate display systems, and in particular to liquid crystal displays (LCD) employing contrast control compensated for variations in power supply voltage and temperature.
Liquid crystal displays (LCDs) are used in many applications as the display of choice because of their small size, low power and low cost. As with any display system, users sometimes want to adjust the contrast between the displayed information and the background. LCDs typically have a contrast input voltage that is used to vary the contrast on a particular display. The contrast is a function of the power supply voltage used on the display and the voltage that is applied to the contrast control pin. The voltage that will generate a particular contrast depends on the display temperature and the actual supply voltage at the time an adjustment was made. If a user sets a contrast level, subsequent variations in the power supply voltage or temperature would require the user to re-adjust the contrast control to maintain the desired contrast.
Many approaches have been implemented in the prior art to deal with the problem of sensitivity of the contrast control setting to variations in power supply voltage and temperature. Some LCD systems try to compensate for only one of the variables while others use rather complex systems of microprocessors, analog to digital (A/D) converters, sensors and feedback systems to compensate for variations that occur when the LCD""s power supply voltage or its ambient temperature vary.
In many LCD systems it is also desirable to have only one voltage to power the display and the circuitry within the display. Having only one power supply voltage can create additional problems in the dynamic range required for contrast control over possible variations in temperature and power supply voltage. Sometimes it is desirable to have a contrast control voltage that is near the level of the display power supply voltage. This dynamic range has led some display system designs to use multiple voltages for the LCD system. As a result, what is needed in the art is a simple and cost effective analog system for providing contrast control for a LCD system using only one supply voltage for the display as well as the contrast control circuitry.
Many modern data processing systems, including but not limited to personal computers, laptop or portable computers use LCDs s as output devices. These data processing systems are operated where it is desirable to have a LCD with an automatic contrast control adjustment. Therefore, the foregoing needs are particularly applicable to such data processing systems that employ a LCD as the primary or as one of the displays for system information.
The present invention addresses the foregoing needs by providing an improved contrast control method and electronic circuitry to implement the contrast control method. More specifically the present invention provides a method where the difference between the display supply voltage and the contrast control voltage are made proportional to a reference voltage which itself is linearly and inversely proportional to temperature. One embodiment of the present invention also uses a novel circuit configuration to enable a high gain and a wide dynamic range for controlling the difference in the display voltage and the contrast control voltage.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.